I wish I would have come across this tool earlier, it would have been a great tool for the numerous presentations that I've done previously. ZoomIt is a nifty little screen zoomer and annotation utility that is very lightweight and handy enough to be used during presentation, especially when you need to enlarge certain areas of a presentation slide OR a demo. It also allows you to annotate the screen to highlight areas or key points.
Careful how you use it though, using too much of it can cause motion sickness in your audience. This tool shouldn't distract you from practicing good presentation design and showing demos with big fonts and clear contrasting colors.